It is well known to oligomerize 1-olefins to hydrocarbons of higher molecular weight and then to hydrogenate or isomerise the oligomer so formed to produce lubricating oils (See e.g.) U.S. Pat. No. 3,763,244. In most of these cases, the 1-olefins are derived initially from ethylene (by the so called "ethylene chain growth and displacement" method) which is a relatively expensive source for such 1-olefins. Moreover, lubricating oils have been produced by oligomerization of relatively pure 1-olefins (see U.S. Pat. No. 3,780,128 and EP-A-0 468 109). This last document also discloses that once the oligomers have been produced, the oligomers of various 1-olefins can be blended either before or after the hydrogenation or isomerization steps in order to produce the lubricating oils of the desired properties such as viscosity index and pour point. One of the problems with this technique of blending is that the final oligomer has a mixture/blend of discrete molecules e.g. a mixture of C20, C30 and C40 hydrocarbons and hence the blend lacks consistency of properties due the absence of a continuity and gradual blending of closely related/matched oligomers. It is also known to oligomerize the olefinic products from a Fischer Tropsch synthesis followed by hydrogenation or isomerization of the oligomer to form lubricating oils (see e.g. Monoolefins, Chemistry & Technology, by F. Asinger, pp 900 and 1089 (1968) and published by Pergamon Press). However, the publications relating to use of the Fischer Tropsch products as the source material for the oligomerization step do not indicate the product mix required to achieve the desired oligomer or the catalyst suitable for the oligomerization step.